Robert M. Olree

How Much ZDP is Enough

Zinc dithiophosphate, or ZDP, for over 60 years has been used as an additive in engine oils to provide wear protection and oxidation stability in an efficient and cost effective manner. Unfortunately, ZDP contains phosphorus, and phosphorus is a widely known and accepted poison of automotive catalysts and other emissions system components. Because of this, phosphorus (and ZDP) levels in automotive engine oils have been gradually reduced by about 35% over the last 10-15 years, and further reductions are likely in the future. This paper traces the history of ZDP use in automotive engine oils, and addresses the issue of how much (if any) ZDP is actually required to provide wear protection in todays, as well as yesterdays, engines. The focus in the paper is on wear (including scuffing) protection, and not on the other aspects of ZDP performance, such as providing oxidation stability of the oil. It is assumed that these other functions of ZDP can be provided by other ashless and phosphorus-free additives which do not negatively affect emissions system performance.